Method and a touch sensing device for implementing the method

ABSTRACT

The present invention relates to a method for determining an acoustic response attributed to a location of an impact, in particular a touch event, on a surface of an object including at least one transducer comprising the steps of: a) receiving an acoustic signal from at least one transducer, wherein the signal corresponds to a predetermined excitation (E) at at least one location on the surface; b) determining an acoustic response based on the acoustic signal received in step a); and c) determining the acoustic response attributed to at least one location on the surface different to the location of the predetermined excitation based on at least two different acoustic responses determined in step b).

The present invention relates to a method for determining an acousticresponse attributed to a location of an impact, like a touch event on asurface of an object, and a touch sensing device implementing themethod.

Methods to determine the location of an impact on a surface of an objectbased on analysing acoustic signals are already known in the art. Thiskind of technology is based on measuring the acoustic signal using oneor more sensors to obtain an acoustic signature of the impact and tocompare the signature with a predetermined set of acoustic signatures,wherein each predetermined acoustic signature is representative for agiven location of the haptic or tactile interface. The location of theimpact is then identified based on the similarity of its acousticsignature with one of the predetermined acoustic signatures. FIG. 1schematically illustrates such a prior art device with an interactionsurface 101 and two transducers 103.

The predetermined set of acoustic signatures, which can be in the formof a lookup table or a database, is generated by computing theindividual acoustic pattern of closely spaced locations 105 on thesurface of a haptic interface during the calibration of the device. Thelookup table is generated by an automated process, e.g. using arobotized bench. The robot used is equipped with a shaker and a metaltip, and performs impacts of a predetermined force and shape on eachlocation sequentially. An acoustic signature for a given location isthus built up from the acoustic responses determined out of the measuredacoustic signals which are received by the at least one sensor after animpact at the location. An acoustic signature is unique to the locationof the impact. The acoustic response is a function of frequency thatrepresents the response of a haptic interface when subjected to amechanical excitation. The acoustic response can be a transfer function,that represents the relation between the mechanical excitation and asensor's output.

WO 2006/015888 A1 simply proposes to store the measured signals of thecalibration stage. WO 2006/069596 A1 proposes to determine acousticsignatures based on correlation of the phases of two acoustic signalsreceived by acoustic sensors when a reference impact occurs on apredetermined area of the interaction interface.

In particular, for touch screen applications, the generation of thelookup table takes a lot of time as typically several thousands ofacoustic responses need to be acquired to cover the whole haptic areawith a sufficient resolution. Furthermore, each product needs to becalibrated individually as, due to the tolerances accepted during themanufacturing process of the object with the haptic surface, oneestablished lookup table cannot necessarily be applied from one objectto the next one. Due to the complexity of certain objects, at least intheir integrated state, a purely theoretical approach to the creation ofthe lookup tables, like proposed by WO 2006/015888, is not alwayspossible.

One way to reduce the calibration time in the industrialized processcould be to use a plurality of calibration stations using robotizedbenches. Due to the relatively high cost of these benches, thecalibration process represents a high investment. Reducing themanufacturing tolerances, on the other hand, would also lead to anunacceptable rise of costs.

It is therefore the first object of the present invention to provide amethod for determining an acoustic response at any location of an impacton a surface of an object and a touch sensing device implementing themethod with which the predetermined set of acoustic signatures can beobtained faster.

Furthermore, it appeared that, due to wear, scratches, aging,temperature variations, integrations drifts or other known or unknowneffects, an object with a tactilised surface changes such that theoriginal lookup table with the predetermined acoustic signatures nolonger provides satisfying results. As a consequence, a recalibration ofthe set of acoustic signatures needs to be carried. This could be doneusing the robotized benches. This does not, however, correspond to avery practical solution.

WO2008/146098 A1 proposes a method to take into account environmentalvariations that can slightly change the size of the sensor. Thethickness of an object has in fact an impact on the wave propagation. InWO2008/146098 A1, a contraction/expansion algorithm is proposed whichcontracts or extends the acoustic responses in the frequency space totake into account a change in thickness to improve the localizationresults. Whereas this method appeared to provide satisfying results forthis special class of variations, it cannot deal with more complexchanges to the object.

It is therefore a second object of the invention to provide a method fordetermining an acoustic response at a location of an impact on a surfaceof an object such that a recalibration of the predetermined set ofacoustic signatures can be carried out in a simplified way.

The first object is achieved with a method for determining an acousticresponse attributed to a location of an impact on a surface of an objectincluding at least one transducer comprising the steps of: a) receivingan acoustic signal from at least one transducer, wherein the signalcorresponds to a predetermined excitation (E) at at least one locationon the surface; b) determining an acoustic response based on theacoustic signal received in step a); c) determining the acousticresponse attributed to at least one location on the surface different tothe location of the predetermined excitation based on at least twodifferent acoustic responses determined in step b). It is the finding ofthis invention that acoustic responses for a given location can bedetermined out of experimental data which relates to other differentlocations. This greatly simplifies the calibration of a touch sensitivesurface of an object, as the number of real impacts on the surfaceduring calibration can be reduced.

In this context, an impact in particular relates to a touch event, likea touch by one or more of a users fingers or a dragging action over thesurface of an object, e.g. a touch screen device. A predeterminedexcitation, which can be of mechanical origin, can also relate to atouch event. Furthermore, the object can be any kind of object made outof any kind of material or material mix as long as acoustic waves can betransmitted and out of any shape. For instance, the object can be madeout of glass, plastic, wood, metals, etc. and the shape can be a flatpanel or a curved panel and, in general, any 3D shape from simple tocomplex. In this context, a transducer corresponds to any means thattransforms an acoustic signal into another kind of signal, e.g. anelectric signal. For instance, a piezoelectric transducer can be used tocarry out the invention. The term “acoustic” relates to frequencies ofup to 100 kHz, in particular up to 40 kHz, more in particular up to 20kHz.

According to one further aspect of the invention, the acoustic signalscan be received from at least two transducers and the acoustic signalsin this case correspond to at least one predetermined excitation at atleast one location on the surface. Thus, in this variant, the at leasttwo different acoustic responses necessary to carry out the inventionare obtained by the presence of at least two transducers. According to avariant, the acoustic signals can be received from at least onetransducer, but the acoustic signals correspond to predeterminedexcitations at at least two locations on the surface. Thus, in thiscase, the different acoustic responses relate to different excitationsand one transducer would be sufficient to carry out the invention. Ofcourse both variants can be combined, thus use of more than onetransducer and use of more than one location at which excitations areexerted on the surface. The amount of different acoustic responses usedin step c) will also depend on the precision of the acoustic responsesdetermined during step c), one wants to obtain.

Preferably, the predetermined excitation can be applied at a locationthat is positioned in a border region of the surface of the object, inparticular in the border region of a part of the surface of the objectserving as a touch sensitive input area or even outside that area.Typically, on an object, only a part of the surface will be used as atouch sensitive input area, e.g. the screen part of a handheldelectronic device. In this case, means for providing the excitations canbe positioned such that they are away from the area of interest, e.g.hidden in the frame region of a screen. Thus, the means for providingthe excitation can actually remain within the object, and be used notonly during calibration but eventually also for recalibrating. In onevariant, the means for providing excitations can be a transducer of thesame type as used for sensing the signals, e.g. a piezoelectrictransducer, but that functions as an actuator to provide excitations.

Advantageously, the method can further comprise a step d) of determiningacoustic responses at a plurality of locations on the surface differentto the at least one location of the predetermined excitation based onthe acoustic response determined in step b). Thus with only a limitedamount of real excitations/impacts, acoustic responses can be determinedfor the whole surface of interest of the object.

Preferably, the method can further comprise a step e) of: gathering theacoustic responses determined at a plurality of locations on the surfacedifferent to the locations of the predetermined excitations in step d)to form a lookup table. Thus the method can be used to establish a setof predetermined acoustic responses as described above in a faster butstill reliable way compared to the prior art using only a limited numberof real impacts. The lookup table comprises data characterizing thesurface of the object and is used during the touch event localizationprocess once the calibration process of the surface has established theset of predetermined acoustic responses. As only a limited number ofreal excitations is necessary to obtain the lookup table for the surfaceof the object, an individual lookup table can be created for eachproduct. This lowers the integration constraints as any deviations fromproduct to product are individually taken into account.

Advantageously, the method can further comprise a step f) of selectingonly acoustic responses, determined during step b) for carrying out stepc), that satisfy predetermined criteria, in particular, concerning atleast one of, signal-to-noise ratio, coherence of the measurements.These criteria are frequency based, thus enable a motivated choice toselect a frequency range for which the acoustic responses are of goodquality. This additional step brings the advantage that the quality ofthe acoustic responses determined during step c) can be furtherimproved.

According to a preferred embodiment, the method can comprise the stepsg) of determining a first representative function based on the distancebetween an auxiliary location, which is positioned away from thelocation of the predetermined excitation, and the locations on thesurface different to the location of the predetermined excitation; h)determining a second representative function based on the distancebetween the auxiliary location and the location of the predeterminedexcitation; i) computing the ratio between the first representativefunction determined in step g) and the second representative functiondetermined in step h); j) repeating step g) for each location on thesurface different to the location of the predetermined excitation andfor each auxiliary location; and k) repeating step h) for each locationof the predetermined excitation. This method based on wave superpositionenables a fully numerical way of determining the desired acousticresponses for locations on the surface that are positioned away from thelocations where excitations occurred.

Preferably, the auxiliary locations can be outside the surface and awayfrom each location of a predetermined excitation and away from eachlocation on the surface different to each location of the predeterminedexcitation. Indeed, by providing the auxiliary locations outside, thealgorithm allows to determine the acoustic properties inside theauxiliary locations.

The second object of the invention is achieved with the methodcomprising, in addition to the method steps of claim 1, a further stepi) of repeating at least steps a) to c), in particular on demand and/orupon detection of particular circumstances and/or on a regular basis. Byrepeating at least steps a) to c) from time to time it becomes possibleto identify changes in the acoustic response and thus also in the set ofpredetermined acoustic signatures which are necessary to identify thelocation of an impact exerted by a user on the tactilized surface of theobject which can thus serve as a touch sensitive interface. By simplyproviding at least one excitation according to step a) of claim 1, themethod allows recalibrating the acoustic responses and effects likewear, scratches, aging, temperature, etc. are taken into account. As are-calibration takes into account the properties of the object at themoment of re-running the calibration, the precision of the updatedacoustic response is better than a process which simply corrects thedatabase by a correction factor. The re-calibration can be automated oron demand, e.g. when a user realizes that the identified locationsdiffer from the real locations.

Advantageously, the method can further comprising a step m) of comparingthe acoustic response with a previously determined acoustic responsecorresponding to that location on the surface, in particular, theacoustic response corresponding to that location on the surface of thelookup table. By keeping a trace of the changes, it will be possible toidentify sources of change of the acoustic response.

Preferably, the predetermined excitations can be provided such that thespacing λ between two neighbouring locations of predeterminedexcitations satisfies the relation λ≦1.2 A, wherein A is

${\frac{2\pi}{\sqrt{\omega}}\left\lbrack \frac{12\left( {1 - v^{2}} \right)\rho}{{Eh}^{2}} \right\rbrack}^{{- 1}/4},$ω is the angular frequency of the acoustic signals, in particular thehighest frequency of interest, from the transducers, E, ν, ρ and h areproperties of the object, being respectively the modulus of elasticity,the Poisson's ratio, the density and the thickness. Even furtherpreferred, the predetermined excitations can be provided such that thespacing λ satisfies the relation 0.5 A≦λ≦1.2 A, in particular, 0.9A≦λ≦1.0 A. In this range, optimized results with respect to performance,thus accuracy of the determined acoustic responses in step c) and theduration of acquisition, can be achieved.

Advantageously, the predetermined excitations can be provided such thatthe number of locations of the predetermined excitations per wavelength,in particular per minimum wavelength of interest, of the acoustic signalis more than one, preferably, between three and six. In this range,optimized results with respect to performance, thus accuracy of thedetermined acoustic responses in step c) and the duration ofacquisition, can be achieved. Here, the minimum wavelength is of courselinked to the maximum frequency of interest defined above.

In the context of the invention, the term “the highest frequency ofinterest” is linked to the ability of a touch event (i.e. touch, drag)to input vibratory energy into the tactile object. Frequencies for whichno energy can be injected by a user do not need to be considered tobuild a lookup table. Typically, the frequency limit is set to about 20kHz. However, to further improve the results one may even considerfrequencies of up to 40 kHz or even up to 100 kHz.

According to a preferred embodiment, the at least one transducer can belocated on the axes of symmetry of the surface of the object. Inparticular, in case the surface is shaped to be rectangular, the atleast one transducer can be located on at least one axes of symmetry ofthe rectangular surface and the predetermined excitation is provided atlocations of every second quadrant of the rectangular surface only. Bydoing so, it is possible to take advantage of the special properties ofthe object and the amount of locations where real predeterminedexcitations have to be carried out in step a) can be reduced. Thus,again, the amount of time required to acquire the associated acousticresponses can be reduced. For other shapes, the situation is similar. Atleast one sensor has to be provided per symmetry axis and an amount ofexcitations needs to be determined for which it is possible to achievethe desired resolution with respect to the acoustic responses determinedduring step c).

Advantageously, the locations of the predetermined excitations can berandomly arranged. By breaking an regular organization of locations atwhich real predetermined excitations are carried out, fictive resonanceswhich may occur during the numeric analysis of the data can be overcome.

Advantageously, the locations of the predetermined excitations can beformed in at least two rows, in particular, such that the spacingbetween the locations in a first row are different with respect to thespacing between the locations in a second row. Preferably, the smallerspacing is on the outer row with respect to the surface of interest.Further preferred, the outer row has at least four times more locationsat which excitations are applied. This has the advantage that againfictive resonances can be avoided.

The object of the invention is also achieved with a computer readablemedium including computer executable instructions stored thereon forperforming the method as described above. With this medium the sameadvantageous as described above can be achieved.

The invention also relates to a touch sensing device according to claim13 comprising: an interaction surface; at least one transducer; and acontrol means for receiving an acoustic signal from at least onetransducer, wherein the signal corresponds to a predetermined excitation(E) at at least one location on the interaction surface; for determiningan acoustic response based on the received acoustic signal; and fordetermining the acoustic response attributed to at least one location onthe interaction surface different to the location of the predeterminedexcitation based on at least two different acoustic responses determinedbased on the received acoustic signal. Thus, with this device, theacoustic response of a certain location necessary to obtain the set ofpredetermined acoustic signatures as mentioned in the introduction, canbe determined out of experimental data which relates to other, differentlocations. This greatly simplifies the calibration of a touch sensitivesurface of an object, as the number of real impacts on the surfaceduring calibration can be reduced. The control system is in particularconfigured to carry out the method as described above.

Preferably, the control means can be configured to receive acousticsignals from at least two transducers and the acoustic signalscorrespond to the predetermined excitation at the at least one locationon the surface or the control means can be configured to receiveacoustic signals from the at least one transducer and the acousticsignals correspond to predetermined excitations at at least twolocations on the surface. As long as either at least two differentexcitations locations or at least two transducers are provided, theadvantages as mentioned above with respect to the method can beachieved.

Preferably, the control means can be configured to determine acousticresponses at a plurality of locations on the surface different to the atleast one location of the predetermined excitation based on the receivedacoustic response. Thus, the entire object, or at least the part of itwhich shall provide a touch sensitive interface can be analyzed andacoustic response data be provided by using the experimental data ofonly some real excitations somewhere on the surface of the object.

According to one advantageous embodiment, the control means can beconfigured to gather the acoustic responses determined at a plurality oflocations on the surface different to the locations of the predeterminedexcitations to form a lookup table corresponding to the locations on thesurface. The input data to carry out the analysis relates to theexperimental data of the excitations. Using this data, it is possible todetermine the acoustic responses for as many locations as necessary forthe applications to be carried out by the device and its touch sensitivesurface. In particular, it is possible to adapt the resolution by simplycalculating the acoustic responses in step c) of the method for morelocations. This only requires additional calculation time and no newexperimental data.

Preferably, the device can further comprise a re-calibration unit forre-determining an acoustic response, in particular on demand and/or upondetection of particular circumstances and/or on a regular basis. There-calibration unit is thus configured to repeat at least steps a) to c)of the method from time to time to identify changes in the acousticresponse. Thus, also the set of predetermined acoustic signatures can beupdated. Therefore negative effects like scratches, wear, aging,temperature, etc. can be taken into account effectively so that thetouch sensitive device's lifetime can be extended.

According to a preferred embodiment, the re-calibration unit can beconfigured to compare the acoustic response of the lookup tablecorresponding to the location on the surface to the one of the newlydetermined acoustic response. Thus the entire lookup table whichcomprises the acoustic signatures can be corrected for any changes tothe acoustic properties but, at the same time, a trace is kept of thechanges to be able to identify the source of any updates, e.g. whetherthey are global or only localized.

Further preferred, the re-calibration unit can comprise at least oneactuator, in particular a piezoelectric transducer, at one location of apredetermined excitation such that the predetermined excitation isapplied at the location of the predetermined excitation by the at leastone actuator.

Preferably, the at least one actuator can be positioned at the border ofa region of interest of the surface of the object. Typically on anobject only a part of the surface will be used as a touch sensitiveinput area, e.g. the screen part of a handheld electronic device. Inthis case, the actuator can be positioned such that it is away from thearea of interests, e.g. hidden in the frame region of a screen, to notjeopardize the user friendliness of the device. Another advantage isthat the at least one actuator can actually remain within the object,and be used for calibration to first establish the acoustic responsesfor the initial lookup table and later also for recalibrating the set ofpredetermined acoustic responses. In one variant, the means forproviding excitations can be a transducer of the same type as used forsensing the signals, e.g. a piezoelectric transducer, but that functionsas an actuator to provide excitations.

The invention also relates to a touch sensitive device calibratingsystem according to claim 16 and in particular comprising the deviceaccording to at least one of claims 12 to 15. This device thus furthercomprises: an input/output unit configured to communicate with the atleast one transducer and/or with the control system; an excitation meansfor providing the predetermined excitation (E) at at least one locationon the surface, wherein the excitation is provided by a mechanicalcontact between the excitation means and the surface; and a positioningmeans configured to locate the excitation means at the at least onelocation on the surface such that the excitation means provides anexcitation at the at least one location on the surface, and wherein thecontrol means is configured to drive the actuator. With this system theadvantageous method described above can be carried out to obtain thelookup table of the acoustic signatures.

Preferably, the input/output unit can be configured to receive anacoustic signal from the at least one transducer.

Advantageous embodiments of the inventive method and the device will bedescribed in the following by referring to the Figures:

FIG. 1 illustrates a two-dimensional schematic view of a touch sensingdevice, as known from the prior art, comprising an interaction surface,wherein locations on the surface of the touch sensing device at whichacoustic responses are acquired are shown (in points);

FIG. 2 illustrates a two-dimensional schematic view of a touch sensingdevice according to a first embodiment of the invention, comprising aninteraction surface, wherein locations on the surface of the touchsensing device at which acoustic responses are experimentally acquiredare shown (in points);

FIG. 3 illustrates the main steps involved in a method for determiningan acoustic response attributed to a location of an impact on thesurface of the touch sensing device of the first embodiment of FIG. 2;

FIG. 4 illustrates steps involved in a method for determining anacoustic response attributed to a location of an impact on the surfaceof the touch sensing device according to a second embodiment;

FIG. 5 illustrates different types of locations on the surface of thetouch sensing device of the present invention playing a role in themethod to determine acoustic responses;

FIG. 6 illustrates locations on the surface of the touch sensing deviceat which predetermined excitations are provided. FIG. 6 a illustrates arandom arrangement of locations of the predetermined excitations andFIG. 6 b illustrates the locations of the predetermined excitationsformed in rows;

FIG. 7 illustrates locations of a transducer comprised in the touchsensing device. FIG. 7 a illustrates the transducer located on the axesof symmetry of the surface of the touch sensing device and FIG. 7 billustrates the transducer located off the axes of symmetry of thesurface of the touch sensing device;

FIG. 8 illustrates an acoustic response interpolation at the locationsof the predetermined excitations;

FIG. 9 illustrates a system for calibrating a touch sensing deviceaccording to a third embodiment of the present invention; and

FIG. 10 illustrates another touch sensing device according to a fourthembodiment of the present invention.

In the following, features and advantageous embodiments of the methodand the device according to the invention will be described in detail.

FIG. 2 illustrates a two-dimensional schematic view of a touch sensingdevice 1 using the method for determining an acoustic response at alocation of an impact on the surface according to the invention. Thetouch sensing device 1 comprises an interaction surface 3, and at leastone transducer 5. In this embodiment, the device comprises twotransducers 5 a, 5 b, e.g. piezoelectric transducers, capacitivepiezoelectric transducers, magnetostrictive piezoelectric transducers,electromagnetic piezoelectric transducers, acoustic velocimeters,accelerometers, optical sensors, microelectromechanical system sensors(MEMs) or any device capable of transforming an acoustic signal into anelectric one. The coupling between the transducer 5 and the interactionsurface 3 is achieved by a fastening device, that can be tape, glue, orthe like. The invention can also be carried out with only one transduceror more than two. The touch sensing device is configured to identify thelocation of a touch event, an impact, a dragging over the surface, andto provide this information to control inputs to an electronic device,like a handheld electronic device, a computer or any kind of machinewith a touch sensitive interface. This functionality can be provided bya control means 7.

To do so, the control means 7 analyzes the acoustic signals receivedfrom the transducer and an acoustic signature is determined. Bycomparing this signature with a set of predetermined signatures storedin a lookup table, the location of the impact on the interaction surface3 is obtained. To overcome the problems in the art which relate to thecreation of the predetermined acoustic signatures as described in theintroductory part and which mainly relate to the fact that each locationon the interaction surface 3, has to be characterized by applyingexcitations to the interaction surface at a plurality of locations 9located all over the interaction surface 3.

In this invention, the control means 7 is configured to determine theacoustic responses which are necessary to determine acoustic signatures,at the locations 9 based on at least one predetermined real excitation Eapplied at a location 11 different to the locations 9. To do so thecontrol means is configured to use the acoustic response A determinedout of the acoustic signal received by the first transducer 5 a and Bdetermined out of the acoustic signal received by the second transducer5 b following the impact at location 11.

In the embodiment with only one transducer, a second excitation at adifferent location than the one illustrated in FIG. 2 needs to beapplied to get sufficient information about the object. Of course, toimprove precision, even more excitations at different locations outsidelocations 9 can be applied.

FIG. 3 illustrates the concept of the corresponding method fordetermining the acoustic response attributed to a location 9 of animpact on the interaction surface 3 of the touch sensing device 1according to the first embodiment.

The method, according to the present invention, comprises the followingsteps of: (STEP S1) receiving an acoustic signal S from at least onetransducer 5 a, 5 b, wherein the signal S corresponds to a predeterminedexcitation E at at least one location 11 on the surface 3; (STEP S2)determining an acoustic response A or B based on the acoustic signal Sreceived in STEP S1; and (STEP S3) determining the acoustic response Xattributed to at least one location 9 on the surface 3 different to thelocation 11 of the predetermined excitation E based on at least twodifferent acoustic responses A, B determined in STEP S2. This method canbe carried out by a computer program product e.g. loaded in the memoryof the control unit 7.

FIG. 4 illustrates a second embodiment of the method for determining theacoustic response X attributed to the location 9 of an impact on theinteraction surface 3 of the touch sensing device 1. Compared to thefirst embodiment, the method according to this embodiment furthercomprises the steps of: (STEP S0) providing predetermined excitations Eat a plurality of locations 11 on the surface 3; (STEP S4) repeatingSTEP S3 to determine the acoustic responses for each location 9 on thesurface 3 different to the locations 11 of the predetermined excitationE such that the acoustic responses attributed to the plurality oflocations 9 on the surface 3 different to the locations 7 of thepredetermined excitation E is determined; (STEP S5) gathering theacoustic responses determined (in STEP S4) at the plurality of locations9 on the surface 3 different to the locations 11 of the predeterminedexcitations E to form a lookup table and forming the set ofpredetermined acoustic signatures.

In the following, one practical way to carry out this method accordingto the invention is described. This embodiment is based on the so calledwave superposition method (WSM) which is a numerical technique foracoustic field reconstruction in radiation and scattering problems. Themain idea is that an acoustic cavity can be replaced by a finite numberof elementary sources of excitation. These sources are located insidethe cavity if the acoustic domain to characterize is outside, andoutside if the acoustic domain to characterize is inside. Thus, theacoustic field of interest is obtained by the summation of the fields ofeach elementary source.

The starting point of the WSM is the following equation:

$\begin{matrix}{{p(r)} = {{j\rho\omega}{\int_{E}{{q\left( r_{s} \right)}{G\left( {r,r_{s}} \right)}{\mathbb{d}{E\left( r_{s} \right)}}}}}} & (1)\end{matrix}$where ρ is the density of the propagation medium and ω is the angularfrequency. The source is denoted q, and its position r_(s) is part ofthe domain E. G is the free field Green's function, which, in this case,can be written as follows:

$\begin{matrix}{{G(x)} = {\frac{1}{\sqrt{x}}{\mathbb{e}}^{{\mathbb{i}}\; k\; x}}} & (2)\end{matrix}$

To convert the analytical equation above to its numerical form, thesources are distributed in a thin shell (3D) or on a continuous contour(2D). By discretizing this geometrical entity in N small segments,normal velocity can be approximated on the surface of the cavity:

$\begin{matrix}{{u_{n}(r)} \approx {\sum\limits_{i = 1}^{N}{Q_{i}{\nabla_{n}{G\left( {r_{s},r} \right)}}}}} & (3)\end{matrix}$where Q_(i) is the volume velocity of a elementary source. Since u_(n)is known, Q_(i) can be written in matrix form as:Q=[D] ⁻¹ u _(n)  (4)with D corresponding to nabla G. We finally get the expression of theacoustic field:

$\begin{matrix}{{p(r)} = {{j\omega\rho}{\sum\limits_{i = 1}^{N}{{G\left( {r_{s},r} \right)}Q_{i}}}}} & (5)\end{matrix}$

Based thereon, one can determine acoustic responses following the methodsteps of FIG. 4, in particular during STEP S3 and S4 at locations whichare away from locations at which real excitations occurred.

To do so, the method first requires the following input parameters: Thelocations 7 on the interaction surface 3 for which the acousticresponses and then the acoustic signatures are stored in the lookuptable or a database. For sake of clarity these locations will be calleddatabase locations in the following. Preferably, they cover essentiallythe entire interaction surface 3.

In addition, the locations of the excitations 11 and their associatedacoustic responses A, B are needed. These will be called excitationlocations 11 in the following. The excitation locations 11 are a set ofpoints where acoustic responses are acquired experimentally following apredetermined excitation E. According to a preferred variant, thesepoints are located in the border region of the interaction surface orjust outside. By optimizing the spacing between excitation locations aswell as their arrangement with respect to each other, the performance,e.g. precision and reduction of calculation power, of the method can beoptimized, as will be described further down.

Furthermore, in carrying out step 3 and 4, additional so calledauxiliary locations 13 are necessary for carrying out the method. Theseauxiliary locations 13 are surrounding the database and the excitationlocations 9 and 11, as depicted in FIG. 5. Preferably, the locations 13are outside the surface 3 and away from the excitation locations 11 ofthe predetermined excitations and the database locations 9 on thesurface 3 different to the locations 11 of the predetermined excitation.

Following STEP S0 of providing predetermined excitations at theexcitation locations 11, one at a time, the acoustic signals for theexcitations locations are determined in STEP S1. In this embodiment, themethod then determines the acoustic response during STEP S2 which is atransfer function, representing the relation between the mechanicalexcitation E (input) and the transducer's 5 a, 5 b responses (output).Thus, the properties of the mechanical excitation/s at locations 11 needto be known. In fact, this can be achieved by having the signals in thetime domain for both the input (excitation) and the output (transducer).On the input side, this can, for instance, be achieved by the presenceof a force sensor directly analyzing the excitation E. Some preferredrealisations of this step will be described later.

Next, the wave number k of the excitation needs to be provided. Thisvalue can be either estimated from the geometry of the tactilizedobject, e.g. a plate shaped object or, for more complex geometries, bedetermined experimentally. Eventually, the estimation can be completedby an experimental part. The general expression of the wave number canbe written as follows:

$\begin{matrix}{k = \frac{\omega}{c}} & (6)\end{matrix}$where ω=2πf is the angular frequency and f is the frequency. c is thespeed at which the waves propagate in the material of the interactionsurface 3 of device 1. In the case where the haptic interface can beconsidered as a thin plate (i.e. the ratio between the length of a sideand the thickness is significantly greater than 1), the wave number canbe expressed as follows:

$\begin{matrix}{k = {\sqrt{\omega}\left\lbrack \frac{12\left( {1 - \upsilon^{2}} \right)\rho}{E\; h^{2}} \right\rbrack}^{1/4}} & (7)\end{matrix}$where E, ν and ρ are material properties of the plate, beingrespectively the modulus of elasticity, the Poisson's ratio and thedensity. h is the plate thickness.

Then (STEP S3 and S4), the determination of the WSM coefficients Q whichwill be applied to the acquired acoustic responses of the excitationlocations 11, requires knowing u_(n) (cf. equation 4). These valuesu_(n) are computed at the auxiliary locations 13, as defined above.Thus, the Green's function (cf. equation 2) is computed between databaselocations and auxiliary locations, these values being u_(n).

The computation of the WSM coefficient can be summarized as follows:

-   -   Repeat for each frequency        -   Repeat for each database location 9            -   Repeat for each auxiliary location 13                -   Determine the distance between the auxiliary                    location 13 and the database location 11                -   Compute the Green's function [P] for this distance                -   Repeat for each excitation location 11                -    Determine a second distance between the auxiliary                    location 13 and the excitation location 11                -    Compute the Green's function [D] for the second                    distance            -   Compute the WSM coefficients [B]=[D]⁻¹[P]

Next, the acoustic responses at database locations 9 are computed byapplying the WSM coefficients to the acoustic responses of theexcitation locations 11.

Finally, according to STEP S5, the acoustic responses previouslycomputed in STEP S3 and S4 are transformed in acoustic signatures thatare unique to each database point. All acoustic signatures associated toall database locations are gathered to form the lookup table.

Before determining the acoustic responses at the database locations 11,an additional step of selecting only relevant frequencies can be appliedas a variant of this embodiment. The acoustic responses acquired atexcitation locations during STEP S2 are functions of frequency.Depending on the acquisition parameters (sampling frequency, framesize), material properties and dimensions of the interaction surface 3,the acoustic responses at some frequencies may be not relevant because,for example, of a poor signal-to-noise ratio.

This additional but not mandatory step thus consists in selectingfrequencies that are considered relevant by applying a criteria based,for example, on the coherence of the measurements. Noisy frequencies arethus rejected, avoiding the performances of the method to be decreased.

In the following, various possibilities to arrange the excitationlocations 11 according to the invention will be described. Thedefinition of location of the excitation locations plays an importantrole in the performances of the method according to the invention.

The main parameters are the geometry, thus how the excitation locationsare arranged, and the appropriate number of excitation locations 11. Byoptimizing these parameters, the goal of having sufficient precision onthe final database to run the touch sensitive device 1 with the desiredresolution concerning the touch sensitive interface 3 with as fewexcitation locations as possible, thus with reduces calculation power,can be achieved.

By applying the following rules and methods, according to the invention,a good compromise between the performances and the number of excitationlocations is achievable.

The geometry formed by the excitation locations 11 can lead to thepresence of fictive resonances during the numerical data treatment.These resonances have no physical meaning and are fully associated tothe numerical formulation. A solution according to the inventionconsists in “breaking” the regular organization of the excitationlocations 11 by locating them randomly along a contour line 15, asdepicted in FIG. 6 a. In particular, the distance and orientationrelationship between the neighbouring locations 11 are random or have noorder.

Another solution, according to the invention, consists in organizing theexcitation locations 11 in a double layer network, as depicted in FIG. 6b. FIG. 6 b shows that the locations of the predetermined excitations 11are formed in at least two rows, in particular, such that the spacingbetween the locations 11 a in a first row are different with respect tothe spacing between the locations 11 b in a second row. The locations 11a in the first row are evenly spaced with respect to each other and thelocations 11 b in the second row are evenly spaced with respect to each.Preferably the row, here 11 a, with more excitation locations, ispositioned towards the database locations 9.

Not only the geometry itself needs to be taken into account, but alsothe spacing between two neighbouring excitation locations 11 has to bechosen. In order to have satisfying performances of the method, thechoice of this spacing preferably follows a criterion based on thewavelength of the highest frequency of interest. If the spacing is toolarge, the acoustic responses in STEP S3 and S4 will only be poorlypredicted. If the spacing is too small, this will have no effect on theperformance of the method, but will lead to a higher number ofexcitation locations 11, and thus a longer acquisition time and a needfor more computation power.

The smallest wavelength, corresponding to the highest frequency ofinterest, is expressed as follows:

$\begin{matrix}{\lambda_{\min} = \frac{2\pi}{k_{\max}}} & (8)\end{matrix}$

Where k_(max) is the wave number (cf. equation 6) of the highestfrequency of interest.

It has been found that three to six excitation locations 11 perwavelength lead to good performances of the method. Below threeexcitation locations 11 per wavelength, the performances tend todecrease dramatically. In the case of the double layer organization ofexcitation locations 11 illustrated in FIG. 6 b, this criterion shouldbe preferably applied on the outer excitation locations 11 b. Regardingthe inner excitation locations 11 a, it has been found that one innerpoint every four outer points lead to good performances of the method.

A further possibility to improve the performance of the method is totake into account symmetry. FIGS. 7 a and 7 b illustrate two of the manypossible arrangements of the transducers 5 on the surface 3 of the touchsensing device 1 in the presence of an symmetry axis. FIGS. 7 a and 7 bshow a rectangular shaped surface 3 comprising the transducers 5. Thetransducers 5 of FIG. 7 a are located at positions away from the axes ofsymmetry 17 of the surface 3. The transducers of FIG. 7 b are located onthe axes of symmetry 17 of the surface 3. When the transducers 5 arelocated on the axes of symmetry 17 of the surface 5, in particular oneon each symmetry axis, the predetermined excitations E can be providedonly at the excitation locations 11 of the predetermined excitations Eof every second quadrant defined by the two symmetry axis 17 of thesurface. Thus, with the benefit of symmetry of the surface 3, it ispossible to reduce the number of locations 11 at which the predeterminedexcitations are provided and, therefore, it is possible to reduce thenumber of acoustic responses that are to be determined during STEP S2.Consequently, the time required to determine the acoustic responses, inSTEP S3, at the locations 9 on the surface different to the locations 11of the predetermined excitations, is reduced.

In this example, it is possible to reduce the number of excitationlocations 11 by a factor 2, thus significantly reducing the timerequired to acquire the associated acoustic responses. However, thisapproach assumes that the boundary conditions of the haptic interfaceare also symmetric. Depending on the shape of the interaction surfaceand the device 1 itself other symmetry rules may apply leading tosimilar effects on the amount of excitation locations 11 depending onthe position of the transducers.

Another way to optimize the performance of the method is to takeadvantage of interpolation methods. They consist in computing acousticresponses on some excitation locations 11 themselves, as depicted inFIG. 8. For example, it is possible that, in the case of a set on N−1excitation locations 11 which respect the frequency criteria set-upabove except between two points 11 a, 11 b (i.e. presence of a “hole” inthe contour), it is possible to estimate the acoustic response of thismissing point 11 c with an acceptable accuracy. This variant can play animportant role in case the method is not only used to create the lookuptable but also to carry out an re-calibration procedure based on theupdate of the set of acoustic responses at the contour point locations.This way actually, for only about one half of excitation locationsacoustic responses, would have to be first acquired to do a partialupdate. Then, interpolation would complete the update by replacingobsolete acoustic responses sequentially, without having to carry outthe experimental excitations for these locations.

Of course the various embodiments described above can be combined in anycombination to further optimize the method according to the invention.

FIG. 9 illustrates the global architecture of a touch sensitive devicecalibrating system 21 according to a third embodiment of the inventionwhich is able to speed up the lookup table/database creation process toobtain the predetermined set of acoustic signatures. The touch sensitivedevice 23 which can be calibrated with system 21, can be the oneillustrated in FIG. 2, thus with a control system configured toestablish the lookup table, but could also be any other one with aninteraction surface 25 and transducers 27, 29 having the same propertiesas above.

The architecture of the system 21 of this embodiment describes a“direct” approach where: the acoustic response of the haptic interface 3is acquired at the same location thus by the same transducers 5, at eachiteration of the process. In contrast thereto, the excitation of thehaptic interface 3 is applied and acquired at different excitationlocations 11 at each iteration of the process. Thus, the direct approachis based on a stationary response and a moving excitation.

A variant of the architecture, not illustrated in FIG. 9, is a so called“reciprocal approach” where: the acoustic response of the hapticinterface 3 is acquired at a different location at each iteration of theprocess and the excitation of the haptic interface 3 is applied andacquired at the same location at each iteration of the process. Thus,the reciprocal approach is based on a stationary excitation and a movingresponse.

The system 21 comprises a front end device with an input/output unit 31,in particular an analog input/output unit 23. The input/output unit 31communicates with the transducers 27, 29 directly or via the controlunit 7 (see FIG. 2) and thus receives the acoustic signals as an input.It may carry out the following actions: signal conditioning of theanalog inputs, like amplification, filtering, automatic gain control,analog-to-digital conversion of the analog inputs, digital-to-analogconversion of analog outputs, signal conditioning of the analog outputs,like amplification and filtering. The unit 31 is furthermore inconnection with an analysing means 33, here a computer, for datatransfer and may provide an interface to configure the system 21.

The system 21 further comprises an excitation means 35 to apply amechanical excitation at an excitation location 37 (corresponding tolocations 11 in the embodiment of FIG. 2) of the interaction surface 25.The excitation means 35 of this embodiment comprises three sub units.

First of all, an excitation device 37, is a tool for applying themechanical excitation E to the interface 25 at an excitation location37. The device 37 in this embodiment comprises at least a rigid tip,like a pike, a small ball made of any rigid material like metals or thelike, to transmit a predetermined excitation upon the interactionsurface 25. Preferably this device 37 may further comprise a device thatconverts an electric signal into a mechanical excitation, like aelectromagnetic shaker, piezoelectric transducers, capacitivepiezoelectric transducers, magnetostrictive piezoelectric transducers,electromagnetic piezoelectric transducer or the like.

The excitation can be applied either by making a contact between the tipand the interaction surface 25 at locations 37 or by moving fast the tipdown and up to briefly strike the interaction surface 25. The excitationdevice 37 is configured to transmit the mechanical excitation in a broadfrequency range, at least corresponding to the range of interest fortouch sensitive applications based on the detection of acoustic signals.A preferred frequency range is the audio frequency ranging from about 0to about 20 kHz, however frequencies of up to 40 kHz or even 100 kHzcould also be considered depending on circumstances.

The excitation unit 35 further comprises an excitation reference device39 configured to provide a measure of the mechanical excitationtransmitted to the interaction surface 25, that is used as an inputreference for the acoustic response acquisition in STEP S2. It can be aforce sensor, a strain gage sensor, an electric signal, or any sensorable to measure a mechanical quantity.

Finally, the excitation means 35 of this embodiment further comprises amoving device 41 to move the excitation device 37 in front of theexcitation location/s 37 where the excitation has to be transmitted tothe interaction surface 25. This device 41 is configured to move fastfrom one location 37 to another, with a high positioning accuracy. Thiscan be achieved with a robot with at least 3 axes of displacement.

The input/output unit 31 and the excitation means 35 are in connectionwith the analysing means 33, here the computer, which controls thesystem 21. The analysing means 33 carries out at least one of thefollowing activities: it provides an interface with the front-enddevice, the input/output unit 31 to configure and to collect data comingfrom the inputs, namely the transducers 27, 29 but also to collect datafrom the excitation reference device 39, to send data to outputs todrive the signal of the excitation device 37, and to communicateinstructions to the moving device 41 to drive the moving device to thecorrect location.

The main task of the analysing means 33 is to apply the method asdescribed above to build up the lookup table. The computer uses themethod presented in this document that enables to determine acousticsignature at any point over the area of the interaction surface, from aset of acoustic responses acquired on the borders of this area.

FIG. 10 depicts the global architecture of a fourth inventiveembodiment. It illustrates another touch sensing device 51 with aninteraction interface 53, with the same properties as in the first tothird embodiments and transducers 55, 57. In addition to the featuresdescribed above with respect to the first and second embodiment (seealso FIG. 1), this device furthermore comprises a re-calibration unit orauto adaptive unit 59.

The touch sensing device 51 of the fourth embodiment also comprises afront-end device 61 which is part of the control means according toclaim 12 and that provides a treatment of the signals received from thetransducers 55, 57, like amplification, filtering, automatic gaincontrol, Analog-to-Digital conversion. The digitized data is provided atan output interface towards the auto-adaptive unit 59 and a back-enddevice 63. Other features of the front-end device could be aconfiguration interface and a power management.

The back-end device 63 is configured to obtain the localization of auser event, like a touch, a drag & drop, . . . , out of the signalspreviously acquired by the front-end device 61 using a technology likedescribed in the introduction above and thus based on the acousticsignals received which are compared with the predetermined set ofacoustic signatures stored in a lookup table or database.

The re-calibration unit or auto adaptive unit 59 fulfils two functionsin this embodiment: First of all, at the first use, an auto-calibrationis carried out to generate the lookup table with the acousticsignatures. This can be done at the first initialisation or any time theback end device 63 instructs the auto adaptive unit 59 to do so. Second,the component is configured to automatically recalibrate the lookuptable of the interaction surface 53 upon identification of a change e.g.due to aging, a certain temperature range, following integration of thedevice into another device or any other change that may affect themechanical and thus the acoustic behaviour of the interaction surface53. The device 51 can also be configured to carry out the re-calibrationupon request from a user or from the back-end device. It could also becarried out regularly, e.g. on a periodical schedule.

The auto-adaptive device 59 uses the method described above with respectto the first and/or second embodiment and which can determine acousticresponses and thus also signatures at any location 65 over the area ofthe interaction surface 53, from a set of acoustic responsesexperimentally acquired on the borders 67 of this area. To be able toapply the predetermined excitations at excitation locations 69, at leastone actuator (four actuators in this embodiment) is attached to thebackside of the interaction surface 53. These actuators convert electricsignals generated by the auto-adaptive device 59 into elastic waves thatpropagate in the interaction surface 53. The actuators 71 can bepiezoelectric transducers, capacitive piezoelectric transducers,magnetostrictive piezoelectric transducers, electromagneticpiezoelectric transducers, or the like, for the transducers 55 and 57which serve as sensors. The coupling between the actuators 71 and theinteraction surface is achieved by a fastening device that can be tape,glue. The auto-adaptive device can be a standard microcontroller andeventually integrated into the front-end device 61, the back-end device63 or on an ASIC chip integrating the three functions.

Finally, the touch sensitive device 51 can be connected to a computer 73or any other suitable electronic device, e.g. a handheld electronicdevice, which receives the locations of touch events or impacts from thetouch sensitive device as input values which can be used to controlapplications of the computer 73.

The auto-calibration feature of the touch sensitive device 51 functionsthe following way: At the first switching on of the system, theauto-adaptive device 51 is in charge to build the lookup table in orderto turn the surface of the object into an touch sensitive interactionsurface 53. This process is the following: a) the auto-adaptive device51 sends a signal to each actuator 71 iteratively to make the objectvibrate, b) the acoustic responses based on the acoustic signals of thetransducers or simply the acoustic signals are iteratively acquiredthrough the front-end device 61 and/or the auto-adaptive device 59 andsent to the auto-adaptive device, based thereon c) the auto-adaptivedevice 59 builds the acoustic signature subset corresponding to theactuator's 71 location, d) the auto-adaptive device 59 builds the lookuptable from the subsets of acoustic signatures determined for allactuator locations 71 and e) the back-end device 63 stores it to itsmemory, finally f) the auto-adaptive device 59 informs the back-enddevice 63 that the system is ready.

The re-calibration feature of the touch sensitive device 51 functionsthe following way. During the whole life of the system, theauto-adaptive device 59 is in charge to monitor the integrity of theinteraction surface 53 to identify potential drifts due to a change inenvironmental conditions and usage. As the back-end device 63 ispermanently waiting for user events, the re-calibration process shouldpreferably run in the background, in order to minimize interruptions ofthe recognition process. For instance upon detection of a drift or atthe lapse of a predetermined recalibration duration, the re-calibrationis trigged. After such a triggering, the back-end device 63 stops andthe auto-adaptive device 59 can send signals to the actuators 71. Infact, the process to re-calibrate and update the lookup table does notneed to be in real time, it can be performed in the background withouthaving to interrupt a user. Thus, the signal emission can be interruptedby the back-end device 63 at any time, e.g. in case of a sudden userevent, and then be continued without losing the re-calibrationcomputation done before the interruption. During re-calibration,essentially the same steps are carried out as during initialization. Toupdate the lookup table, the old signatures are overwritten with the newones. Eventually a trace can be kept of the changes to be able toanalyze the changes, e.g. to identify the source of a change in thelookup table.

The embodiments of this invention provide a method that is used todetermine acoustic signatures at any point over an area to be turnedinto a sensitive object, from a set of acoustic responses acquired atother locations, in particular on the borders of this area. With thisinventive concept, the following advantages compared to the prior artcan be achieved: It can reduce the number of acoustic responsesnecessary to acquire the acoustic signatures, namely typically to a fewhundreds or even less instead of several thousand, it enables the fastacquisition of the lookup table, namely a few minutes instead of severalhours, it furthermore enables to have a dedicated lookup table for eachpiece and it functions with any kind of material (glass, plastic, wood,metals, etc.) and shape (flat & curved panels, complex 3D surfaces). Inaddition, with the method, the integration constraints (as each piecehas its dedicated lookup table, sensitivity to integration constraintsis lower) can be lowered. A further major advantage is that it becomespossible to carry out auto-calibration (i.e. it is capable of buildingits own lookup table) and re-calibration (i.e. it is capable of adaptingits own lookup table if required, to compensate, for example, wear,scratches, aging, temperature, integration drifts).

The third and fourth embodiment are explained for two transducers,nevertheless the invention can be carried out with only one transduceror more than two.

The invention claimed is:
 1. A method for determining an acousticresponse attributed to a location of an impact, in particular a touchevent, on a surface of an object including at least one transducercomprising the steps of: a) receiving an acoustic signal from the atleast one transducer, wherein the signal corresponds to a predeterminedexcitation (E) at at least one location on the surface; b) determiningan acoustic response based on the acoustic signal received in step a);c) determining the acoustic response attributed to at least one locationon the surface different to the location of the predetermined excitationbased on at least two different acoustic responses determined in stepb); g) determining a first representative function based on the distancebetween an auxiliary location, which is positioned away from thelocation of the predetermined excitation and the locations on thesurface different to the location of the predetermined excitation, andthe location on the surface different to the location of thepredetermined excitation; h) determining a second representativefunction based on the distance between the auxiliary location and thelocation of the predetermined excitation; i) computing a ratio betweenthe first representative function determined in step g) and the secondrepresentative function determined in step h); j) repeating step g) foreach location on the surface different to the location of thepredetermined excitation and for each auxiliary location; and k)repeating step h) for each location of the predetermined excitation. 2.The method according to claim 1, wherein the acoustic signals arereceived from at least two transducers and the acoustic signalscorrespond to at least one predetermined excitation at at least onelocation on the surface.
 3. The method according to claim 1, wherein theacoustic signals are received from at least one transducer and theacoustic signals correspond to predetermined excitations at at least twolocations on the surface.
 4. The method according to claim 1, whereinthe predetermined excitation is applied at a location that is positionedin a border region of a part of the surface of the object serving as atouch sensitive input area or outside the border region.
 5. The methodaccording to claim 1, further comprising a step d) of: determiningacoustic responses at a plurality of locations on the surface differentto the at least one location of the predetermined excitation based onthe acoustic response determined in step b).
 6. The method according toclaim 5, further comprising a step e) of: gathering the acousticresponses determined at a plurality of locations on the surfacedifferent to the locations of the predetermined excitations in step d)to form a lookup table.
 7. The method according to claim 6, furthercomprising a step m) of comparing the acoustic response with apreviously determined acoustic response corresponding to that locationon the surface of the lookup table.
 8. The method according to claim 1,further comprising a step f) of: selecting only acoustic responses,determined during step b) for carrying out step c), that satisfypredetermined criteria including at least one of signal-to-noise ratioor coherence of the measurements.
 9. The method according to claim 1,wherein the auxiliary locations are outside the surface and away fromeach location of a predetermined excitation and away from each locationon the surface different to each location of a predetermined excitation.10. The method according to claim 1, further comprising a step l) ofrepeating steps a) to c) based on at least one of an on demand basis ora scheduled basis.
 11. The method according to claim 1, wherein thepredetermined excitations are provided such that the spacing λ betweentwo neighboring locations of predetermined excitations satisfy therelation λ≦1.2 A wherein A is proportional to${\frac{2\;\pi}{\sqrt{\omega}\;}\left\lbrack \frac{12\left( {1 - v^{2}} \right)\rho}{{Eh}^{2}} \right\rbrack}^{{- 1}\text{/}4},$ω is the angular frequency of the acoustic signals from the transducers,E, ν, ρ, and fi are properties of the object, being respectively modulusof elasticity, the Poisson's ratio, density and thickness of the object.12. The method according to claim 11, wherein the predeterminedexcitations are provided such that the spacing λ satisfy the relation0.5 A≦λ≦1.2 A.
 13. The method according to claim 11, wherein: ω is amaximum frequency of interest; and the spacing λ between two neighboringlocations of predetermined excitations satisfy the relation 0.5 A λ≦1.2A.
 14. The method according to claim 1, wherein the predeterminedexcitations are provided such that the number of locations of thepredetermined excitations per wavelength of the acoustic signal is morethan one.
 15. The method according to claim 1, wherein the at least onetransducer is located on an axes of symmetry of the surface of theobject.
 16. The method according to claim 15, wherein the surface isshaped to be rectangular, the at least one transducer is located on atleast one axes of symmetry of the rectangular surface and thepredetermined excitation is provided at locations of every secondquadrant of the rectangular surface.
 17. The method according to claim1, wherein the locations of the predetermined excitations are randomlyarranged.
 18. The method according to claim 1, wherein the locations ofthe predetermined excitations are formed in at least two rows such thatthe spacing between the locations in a first row are different withrespect to the spacing between the locations in a second.
 19. Anon-transitory computer readable medium including computer executableinstructions stored thereon that when executed by a processor configurethe processor to: a) receive an acoustic signal from at least onetransducer, wherein the signal corresponds to a predetermined excitation(E) at at least one location on a surface of an object including the atleast one transducer; b) determine an acoustic response based on theacoustic signal received in step a); and c) determine the acousticresponse attributed to at least one location on the surface different tothe location of the predetermined excitation based on at least twodifferent acoustic responses determined in step b); g) determine a firstrepresentative function based on the distance between an auxiliarylocation, which is positioned away from the location of thepredetermined excitation and the locations on the surface different tothe location of the predetermined excitation, and the location on thesurface different to the location of the predetermined excitation; h)determine a second representative function based on the distance betweenthe auxiliary location and the location of the predetermined excitation;i) compute a ratio between the first representative function determinedin step g) and the second representative function determined in step h);j) repeat step g) for each location on the surface different to thelocation of the predetermined excitation and for each auxiliarylocation; and k) repeat step h) for each location of the predeterminedexcitation.
 20. A touch sensing device comprising: an interactionsurface; at least one transducer; and a controller configured to: a)receive an acoustic signal from at least one transducer, wherein thesignal corresponds to a predetermined excitation (E) at at least onelocation on the interaction surface; b) determine an acoustic responsebased on the received acoustic signal; c) determine the acousticresponse attributed to at least one location on the interaction surfacedifferent to the location of the predetermined excitation based on atleast two different acoustic responses determined based on the receivedacoustic signal; g) determine a first representative function based onthe distance between an auxiliary location, which is positioned awayfrom the location of the predetermined excitation and the locations onthe surface different to the location of the predetermined excitation,and the location on the surface different to the location of thepredetermined excitation; h) determine a second representative functionbased on the distance between the auxiliary location and the location ofthe predetermined excitation; i) compute a ratio between the firstrepresentative function determined in step g) and the secondrepresentative function determined in step h); j) repeat step g) foreach location on the surface different to the location of thepredetermined excitation and for each auxiliary location; and k) repeatstep h) for each location of the predetermined excitation.
 21. Thedevice according to claim 20, wherein the controller is configured toreceive acoustic signals from at least two transducers and the acousticsignals correspond to the predetermined excitation at the at least onelocation on the surface or wherein the controller is configured toreceive acoustic signals from the at least one transducer and theacoustic signals correspond to predetermined excitations at at least twolocations on the surface.
 22. The device according to claim 20, whereinthe controller is configured to determine acoustic responses at aplurality of locations on the surface different to the at least onelocation of the predetermined excitation based on the received acousticresponse.
 23. The device according to claim 20, wherein the controlleris configured to gather the acoustic responses determined at a pluralityof locations on the surface different to the locations of thepredetermined excitations to form a lookup table corresponding to thelocations on the surface.
 24. The device according to claim 20, furthercomprising: a re-calibration unit for re-determining an acousticresponse based on at least one of an on demand basis or a scheduledbasis.
 25. The device according to claim 24, wherein the re-calibrationunit is configured to compare the acoustic response of the lookup tablecorresponding to the location on the surface to the re-determinedacoustic response.
 26. The device according to claim 24, wherein there-calibration unit further comprises at least one actuator at thelocation of the predetermined excitation such that the predeterminedexcitation is applied at one location of a predetermined excitation bythe at least one actuator.
 27. The device according to claim 26, whereinthe at least one actuator is positioned at the border of a region ofinterest of the surface of the object.
 28. A touch sensitive devicecalibrating system, comprising: a touch sensitive device including: aninteraction surface; at least one transducer; and a controllerconfigured to: a) receive an acoustic signal from at least onetransducer, wherein the signal corresponds to a predetermined excitation(E) at at least one location on the interaction surface; b) determine anacoustic response based on the received acoustic signal; c) determinethe acoustic response attributed to at least one location on theinteraction surface different to the location of the predeterminedexcitation based on at least two different acoustic responses determinedbased on the received acoustic signal; g) determine a firstrepresentative function based on the distance between an auxiliarylocation, which is positioned away from the location of thepredetermined excitation and the locations on the surface different tothe location of the predetermined excitation, and the location on thesurface different to the location of the predetermined excitation; h)determine a second representative function based on the distance betweenthe auxiliary location and the location of the predetermined excitation;i) compute a ratio between the first representative function determinedin step g) and the second representative function determined in step h);j) repeat step g) for each location on the surface different to thelocation of the predetermined excitation and for each auxiliarylocation; and k) repeat step h) for each location of the predeterminedexcitation; an input/output unit configured to communicate with one ormore of the at least one transducer and the controller; an excitationmeans for providing the predetermined excitation at the at least onelocation on the surface, wherein the predetermined excitation isprovided by a mechanical contact between the excitation means and thesurface; and a positioning means configured to locate the excitationmeans at the at least one location on the surface such that theexcitation means provides an excitation at the at least one location onthe surface.
 29. The device according to claim 28, wherein theinput/output unit is configured to receive the acoustic signal from theat least one transducer.